Background: Thirty-one gliosarcomas (25 nonirradiated and 6 postirradiated tumors) were selected based on the presence of two distinctive areas: a malignant gliomatous and a sarcomatous component. In all cases, the sarcomatous component appears like fibrosarcoma or malignant fibrous histiocytoma. Two tumors showed additional areas consisting of osteochondroid differentiation. Methods: All tumors were examined using antibodies against Ulex europaeus agglutinin I (UEA), glial fibrillary acidic protein (GFAP), vimentin (VM), epithelial membrane antigen (EMA), desmin, collagen IV, alpha-1-antitrypsin (a-l-AT) and smooth muscle actin (SMA). Results: While the VM highlighted the sarcomatous areas of all tumors there were only scattered spindle cells that were GFAP-positive in the same areas. The a-l-AT was diffusely reactive in the sarcomatous areas in 20 cases. Focal immunoreactivity was seen with SMA (20 tumors), UEA (8 tumors), EMA (5 tumors), collagen IV (5 tumors) and desmin (4 tumors) in the nonvascular sarcomatous cells. Conclusions: The range of immunophenotypical expression is likely to be a reflection of the capacity of a multipotential progenitor to undergo divergent differentiation. There is very little morphological difference between the postirradiated and nonirradiated tumors except that a higher proportion of postirradiated tumors are immunoreactive to SMA and desmin.